An estimated 3.9 million Americans and 170 million individuals worldwide have been infected with hepatitis C virus (HCV). Chronically infected individuals are at risk of developing severe and life-threatening liver disease, and HCV is the leading cause of liver transplantation in the U.S. No vaccine for HCV is currently available, and the existing therapies are non-specific antiviral agents with modest efficacies and significant toxicities. Therefore, there is an urgent need for new, targeted HCV therapies. Entry inhibitors currently are used to treat other viral infections; however, until recently there were limited systems for studying entry and inhibition of HCV. A major advance was the recent discovery that the HCV envelope glycoproteins E1E2 can pseudotype retroviral core particles. These HCV pseudovirus particles (HCVpp) accurately recapitulate the essential biology of HCV entry and provide for the first time a robust means to elicit and screen panels of monoclonal antibodies (MAbs) for HCV-neutralizing activity. A second landmark development was the discovery of a virulent strain of HCV that replicates efficiently in cell culture (HCVcc), enabling MAbs to be tested against authentic virus in vitro. In this new Phase I project, we seek to leverage these important discoveries for the purposes of developing novel and broadly neutralizing MAbs (NtMAbs) for the treatment of HCV infection. Fusogenic HCVpp will be used to immunize mice using novel regimens. Approximately 25,000 hybridomas will be generated and analyzed for specific inhibition of HCVpp using state-of-the-art screening. The most promising NtMAbs will be tested in purified form against a panel of =50 HCVpp representing the major HCV genotypes and against the available strains of HCVcc. In addition, NtMAbs will be mapped for E1E2 binding and tested for activity against unrelated viruses to establish their specificity. Project success requires that we identify a novel NtMAb that is superior to all existing NtMAbs in terms of potency and spectrum of anti-HCV activity. A NtMAb that meets our stringent criteria for development would represent both a significant advance to this field and an important new HCV drug candidate. In the Phase II project, the lead NtMAb will be evaluated for efficacy in established in vivo models of HCV infection and will be humanized to support clinical testing as a novel mode of HCV therapy. [unreadable] [unreadable] Approximately 170 million individuals worldwide are infected with hepatitis C virus (HCV). Chronically infected individuals are at risk of developing severe and potentially life-threatening liver disease, including cirrhosis and hepatocellular carcinoma. Current therapies have modest efficacies and significant toxicities, and there is an urgent need for new therapies to combat HCV infection. [unreadable] [unreadable] [unreadable]